1. Field of the Invention
The invention relates to optical devices; more particularly, it relates to optical circulators.
2. Description of Related Art
An optical circulator is a nonreciprocal, typically three-port or four-port, device. Light entering the first port passes out the second port, but light entering the second port can not pass back to first port. Instead, it passes out of the third port. By installing an optical circulator at each end of a fiber link, an existing unidirectional fiber optic communication link can be quickly and economically converted to a bi-directional one. Such a modification results in a doubled bit carrying capacity. An optical circulator can also be used in applications such as wavelength division multiplexer (WDM), Erbium-doped fiber amplifier (EDFA), add-drop multiplexers, dispersion compensators and optical time domain reflectometers (OTDR's).
Optical circulators are a key element in today's optical networks. However, it has not been widely adopted because of its high cost. A typical optical circulator usually comprises many optical elements and has a large optical footprint. Manufacturing of conventional optical circulators usually requires precise alignment of each optical element, leading to low yields and high production costs.
An early concept of a polarization independent optical circulator for telecommunication use was disclosed in Matsumoto, U.S. Pat. No. 4,272,159. This document, and all others referred to herein, are incorporated by reference as if reproduced fully herein. Such circulators are widely used in conventional WDM optical networks.
Optical circulators have been described in patents, including the above-mentioned Matsumoto, U.S. Pat. No. 4,272,159; Emkey, U.S. Pat. No. 4,464,022; and Kuwahara, and U.S. Pat. No. 4,650,289. However, these early optical circulators often suffer from high insertion loss and/or cross-talk that is unacceptably high for many communications applications. Insertion loss is defined as the difference between the power between light launched into the optical circulator and the power that exits the device. Insertion loss is largely due to coupling loss from fiber to fiber, absorption of light and to imperfect polarization separation. Cross-talk in an optical circulator refers to the amount of power emitted at port 3 (to the receiver) from light entering at port 1 (from the transmitter). The conventional polarizing cubes used in these prior optical circulators often cause large insertion loss and cross-talk because of its low polarization extinction ratio.
Recent circulators as described in Koga, U.S. Pat. No. 5,204,771; and U.S. Pat. No. 5,319,483 represent some improvement in either insertion loss or cross talk. The optical circulators of Koga involve a beam path determining means for introducing a beam to a different direction depending upon the direction of the electric field vector and the propagation. In the context of the Koga circulators, the different direction being referred to is really an example of beam shifting, rather than a change in propagation direction. The beam path determining means of Koga shifts a beam such that it possesses the same propagation direction, but is spatially located in a different portion of the circulator. In this sense, the input beam to and output beam from the beam path determining means are parallel in propagation direction but are shifted in spatial location. A disadvantage of the Koga circulators is that the construction of these circulators demands precise fabrication of birefringent crystals and precise matching waveplates. These types of circulators are therefore often difficult and costly to make. The size of these circulators is also excessively large.
Recent circulators disclosed in Cheng, U.S. Pat. No. 5,471,340; and Cheng, U.S. Pat. No. 5,574,596, appear to have low insertion loss and cross talk, and also to have a simplified structure compared to previous circulators. The Cheng patents disclose the use of beam path determining means of several long birefringent crystals to replace conventional polarizing cubes. The beam path determining means serve to shift the beams passing through, rather than changing the propagation direction. A disadvantage of the Cheng circulators is that long birefringent crystals are often difficult to fabricate. They are also quite expensive. Furthermore, the footprint of this type of circulator is more bulky than other circulators.
Another, fairly serious, drawback of the Cheng circulators is that polarization mode dispersion ("PMD") in the circulators is not eliminated unless additional compensation crystals are introduced. Such additional crystals add significant cost and complexity. Polarization mode dispersion is introduced in an optical component when signal energy at a given wavelength is resolved into two orthogonal polarization modes of slightly different propagation velocity or optical path. The resulting difference in propagation time between polarization modes is called differential group delay or PMD. PMD causes a number of serious capacity impairments, including pulse broadening. In addition, alignment of this type of circulators depends on sub-micron precision positioning of single mode fibers. Therefore, manufacturing of the PMD corrected Cheng circulators is non-trivial.
Pan, U.S. Pat. No. 5,689,593, describes another type of circulator that uses several polarization beam splitting prisms to arrange all optical ports on one side and eliminate polarization mode dispersion. This design has the convenience of having all optical ports on one side. Additionally, polarization mode dispersion is eliminated by symmetry of the optical path. However this design uses several polarization beam splitter cubes, making it very bulky and expensive.
Given the above-mentioned problems with prior art optical circulators, there is a need for a simplified optical circulator comprised of simple optical elements with reduced polarization mode dispersion that is suitable for volume manufacturing.